The present invention concerns a process for the production of purified water and hydrocarbons from Fischer-Tropsch synthesis. This process makes it possible to eliminate the impurities present in the water produced during this synthesis, in particular water-soluble oxygenated compounds, as well as the metals generally produced by the catalyst used in the synthesis stage.
The Fischer-Tropsch synthesis is a process which simultaneously produces water and hydrocarbons. For example when the fossil resource under consideration is natural gas mainly constituted by methane, the global reaction can be written:
nCH4+n/2 O2xe2x86x92n(xe2x80x94CH2xe2x80x94)+n H2Oxe2x80x83xe2x80x83(1)
The quantities of water co-produced are considerable. Thus an industrial unit producing 500,000 t distillates per year co-produces approximately 600,000 t water per year, i.e. approximately one barrel of water per barrel of hydrocarbons.
Fischer-Tropsch synthesis consists, in an initial stage, of transforming the fossil resource (for example natural gas, or a naphtha cut or a heavy petroleum cut, or coal) into synthesis gas, i.e. into a gaseous mixture containing carbon monoxide, carbon dioxide and hydrogen.
In the case of natural gas, gas production processes are generally used which involve synthesis by partial oxidation or steam reforming, or else by a combination of the two aforementioned technologies. When the fossil resource is coal, lignite, asphalts or residues of petroleum origin, gasification or partial oxidation processes are used to produce the synthesis gas.
In a second stage the synthesis gas is converted into water and hydrocarbons according to the reaction:
nCO+2n H2xe2x86x92n(xe2x80x94CH2xe2x80x94)+n H2Oxe2x80x83xe2x80x83(2)
It is known that this reaction, which is strongly exothermic, is generally carried out in the presence of catalysts essentially comprising the metals iron or cobalt deposited on a support phase based on metal oxides such as aluminium, silicon or titanium oxide. Said catalysts are used in various types of reactors, for example multi-tubular isothermal reactors or fluidised bed reactors. In a preferred process, the said catalysts are used in suspension in a liquid phase made up of hydrocarbons (xe2x80x9cslurryxe2x80x9d technology).
In a third stage the hydrocarbons formed are converted partly by means of a hydro-isomerising hydro-cracking process carried out in the presence of a catalyst. Liquid hydrocarbon cuts, for example medium distillates, oils and gases are obtained, which contain slightly isomerised products to improve their properties during use: flow point for the gas-oil and kerosene cuts, viscosity index for lubricants.
The water co-produced with the hydrocarbons is partly recycled in the synthesis gas production section, in particular when the said synthesis gas is produced by steam-reforming, and partly rejected following sufficient treatments to bring it up to standard (e.g. conformity to the chemical oxygen demand). The expert knows that said water contains numerous water-soluble oxygenated compounds (e.g. alcohols, aldehydes, ketones, acids, esters, acetates, aldols etc.), traces of hydrocarbons as well as traces of heavy metals from the catalyst (e.g. from the iron or cobalt) in concentrations generally ranging from 10 parts per million (ppm) to 0.001 ppm. The oxygenated compounds and the metals present in this water give it an odour, a taste, and a potential toxicity incompatible with use for sanitary purposes or for consumption.
Apart from water, the Fischer-Tropsch process co-produces carbon dioxide (CO2) in a fairly large quantity. The CO2 co-produced, apart from that originating from the oven combustion gases, comes from the synthesis gas generation stage and the Fischer-Tropsch synthesis itself (reaction 2 above) and the carbon dioxide conversion reaction (Water Gas Shift, reaction 3 below), whose degree of advance depends in particular on the nature of the catalyst used for the hydrocarbon synthesis (reaction 2).
m CO+m H2Oxe2x86x92m CO2+m H2xe2x80x83xe2x80x83(3)
Part of the CO2 can be recycled to the synthesis gas production unit but, overall, all the oxygen present in the synthesis gas is found in the form of water or CO2 in the reaction products.
The expert also knows that the used waters can be effectively purified by a series of processes, some of which use adsorption on solids, and others biodegradation on dedicated strains of bacteria. The patent U.S. Pat. No. 5,569,790 describes a process for the purification of water used for washing a hydrocarbon charge involved in an ether production process. This purification is carried out by means of liquid-liquid contact with an amount of effluent arising from an etherification unit. In this process, the degree of purification of the water is limited, as the division of the impurities is carried out in favour of the aqueous phase. Moreover, such a process does not enable metals to be eliminated correctly.
The patent FR-2.772.373 describes a process for purification of a fluid, and in particular a process for the elimination of polar compounds contained in the washing water produced by an etherification unit. This process consists of bringing the fluid to be purified into contact with a vapour phase, to get rid of the majority of the impurities. The purification can be carried out by passing the fluid over a solid phase chosen to retain the impurities. The possibility of eliminating the oxygenated compounds and the metals simultaneously is neither described nor mentioned in this application.
The invention concerns a process for the production of purified water and hydrocarbons resulting from Fischer-Tropsch synthesis, comprising at least one stage of separation of the water and hydrocarbons formed during the Fischer-Tropsch synthesis, at least one stage of purification of the separated water by means of contact with at least one adsorbent selected from the group consisting of: the active carbons, clays which are hydrophobic or rendered hydrophobic, and zeolites which are hydrophobic or rendered hydrophobic. This process may in addition include a stripping stage before the purification by adsorption.
It has been found that by means of a simple technique involving adsorption on solids, it is possible to simultaneously eliminate oxygenated compounds, hydrocarbons and also the heavy metals present in the water co-produced with the hydrocarbons in the Fischer-Tropsch synthesis and thus to confer upon it usage properties compatible with use for sanitary purposes and, preferably for consumption.
It has also been found that water for consumption thus produced can possibly, after adjustment of its mineralisation, be gasified, partly using the CO2 resulting from at least one of the stages of the Fischer-Tropsch process.
The process according to the invention consists of separating the water formed from the hydrocarbons co-produced in the Fischer-Tropsch synthesis, possibly condensing this water after separation, then purifying said water by bringing it into contact with at least one adsorbent, selected from the group consisting of: the active carbons, clays which are hydrophobic or rendered hydrophobic, and zeolites which are hydrophobic or rendered hydrophobic. The clays or zeolites can be rendered hydrophobic by any means known to a man skilled in the art, e.g. by silicon grafting or dealumination.
The adsorbent according to the invention is preferably selected from the group consisting of: dealuminated zeolites with an Si/Al atomic ratio greater than or equal to ca. 20, silicalite, and clays which are hydrophobic or rendered hydrophobic. In a more preferred process the dealuminated zeolites in this group are either at least one dealuminated ZSM-5 zeolite, or at least one dealuminated Y zeolite, or a mixture of these two types of zeolite. In a highly preferred process, the adsorbent is a ZSM-5 zeolite with an Si/Al atomic ratio greater than or equal to ca. 20 or a dealuminated Y zeolite with an Si/Al atomic ratio greater than or equal to ca. 20 or a mixture of these zeolites with an Si/Al atomic ratio greater than or equal to ca. 20. Such an adsorbent makes it possible to simultaneously eliminate the different types of impurity contained in the water resulting from the Fischer-Tropsch synthesis, i.e. in particular oxygenated compounds and heavy metals.
When the adsorbent is a zeolite, it is in fact preferable for its Si/Al atomic ratio to be greater than or equal to ca. 20, more preferable for it to be greater than or equal to 50, and highly preferable for it to be greater than or equal to ca. 100.
The adsorption stage is carried out by bringing the water into contact with the adsorbent placed in a reactor. Any type of reactor known to the expert can be used. The adsorption is generally effected at a temperature below ca. 50xc2x0 C., more preferably below ca. 40xc2x0 C., and highly preferably below ca. 30xc2x0 C., and especially preferably below ca. 25xc2x0 C. The mass of water treated per unit of mass of adsorbent and per hour is generally at least equal to approximately 5 hxe2x88x921, preferably generally less than approximately 3 hxe2x88x921, and highly preferably less than 1 hxe2x88x921.
Having reached saturation of said adsorption mass, the flux of water to be purified may be guided towards a second reactor and the saturated adsorbent may be regenerated by any technique known to a man skilled in the art, for example by stripping with water vapour at a temperature of at least 110xc2x0 C., preferably 130xc2x0 C., and highly preferably 150xc2x0 C., so as to reduce the adsorbate content of said mass at least down to 1%, and preferably less than 0.5% of the mass.
Moreover, unexpectedly, the gaseous flow obtained during stripping with water vapour and containing the aforementioned impurities can be converted into hydrocarbons which can be further utilised. This is preferentially achieved by recycling all or part of the said flux towards the synthesis gas production section necessary for the hydrocarbon production stage.
According to a preferred method for implementing the procedure according to the invention, another stage of purification can be effected before the water purification treatment by adsorption. This stage consists of partially purifying the said water by stripping under a pressure preferably between 0.02 and 10 MPa, preferably with an inert gas or medium-pressure water vapour, i.e. water vapour or an inert gas at a pressure generally above 0.02 MPa and below 10 MPa, more preferably between 0.02 MPa and 5 MPa, and highly preferably between 0.02 MPa and 3 MPa. This water vapour or inert gas are used at a temperature generally within the range 60xc2x0 C. to 210xc2x0 C., preferably within the range 80xc2x0 C. to 200xc2x0 C. and more preferably close to 150xc2x0 C., with a water vapour or inert gas to liquid water ratio in terms of mass preferably above or equal to 10, more preferably above or equal to 7, and highly preferably above or equal to 5, for a period sufficient to eliminate at least 30%, preferably at least 50%, highly preferably at least 70% and particularly preferably at least 80% of the aforementioned organic impurities.
It may be possible to inject at the stripping stage a dilute alkali or acid to maintain the pH within an approximately neutral range. The flux of vapour produced by the stripping may also be advantageously recycled to the synthesis gas production section.
After treatment of the impure water by adsorption and possibly pretreatment by stripping, it may be advantageous to post-treat the purified water by means of at least one biological process, e.g. by percolation on a bacterial bed and/or treatment on activated sludge according to technologies known to the man skilled in the art.
The purified water produced by means of the process according to the invention can be used, depending on its degree of purification, either for irrigation of dry geographical area, or for sanitary use, or for consumption.
When used for consumption, the water purified by means of the process according to the invention can advantageously be brought into contact with air, by means of any known process. It can also be advantageously mineralised, for example by the addition of carbonates, hydrogenocarbonates, sulphates, chlorides, phosphates, and metals selected from the group consisting of sodium, potassium, calcium and magnesium, without this list being exhaustive, in proportions known to the man skilled in the art.
Finally, the purified water, preferably drinkable, produced by means of the process according to the invention may possibly be gasified by being brought into contact, under slight pressure (preferably below 0.5 MPa) with part of the CO2 co-produced and purified in advance, in particular to remove the carbon monoxide, by any process known to the man skilled in the art.